1. Field of the Invention
This invention is directed to a rivet making method and the resultant rivet, and particularly a high-strength clad corrosion resistant rivet.
2. Prior and Related Art
One of the most troublesome and severe corrosion problems confronting the aircraft industry today is the exfoliation of aircraft structural skin material, especially high-strength aluminum alloy aluminum-coated sheets, starting at the fastener-receiving holes.
Metal cladding or painting of the exterior surfaces of the aircraft does not eliminate this difficulty. When the holes for the fasteners are drilled or punched in the structural members or plates, to receive the fasteners, the cladding no longer provides the desired protection for the end-grain of the high-strength base metal alloy which is then exposed in the walls of the fastener-receiving holes. Moisture seeps or is drawn into the fayed surfaces or between the fasteners and the walls of the holes and countersunk openings where the end-grain of the structural material or sheets was exposed by the drilling (or punching), for the reception of the fasteners.
With paint coatings, when applied to the skin surfaces, moisture penetration may be retarded or postponed to some degree but once the coating becomes aged it begins to crack or flake around the fastener heads. This allows the moisture direct access to the critical end grain areas in the walls of the holes around the fasteners. In either case, although painting has been preciously recommended, since it afforded some degree of temporary protection, moisture was found to eventually penetrate, and corrosion and exfoliation occurred.
Also fastener installation with a wet zinc chromate primer, or uncured fuel tank sealant has been used but this did not produce the desired results, particularly, it does not solve the problems of nonelectrical continuity and temperature variations.
The teachings of Dalton U.S. Pat. No. 3,642,312 do not properly overcome the corrosion problem. That patent teaches the making of a rivet of high-strength aluminum alloy and then apply a pure, soft aluminum coating to the exterior of the already made rivet. The patent teaches that this coating may be applied by mechanical cladding, such as jacketing and peening. It also suggests that flame spraying, vacuum deposition, molten dipping, electroplating and vapor or gas plating can be used. It tells us that any method may be of value so long as a relatively thick and soft coating of pure aluminum is applied so that the fastener insertion application technique will move or smear some of the coating from the exterior of the rivet onto and into the exposed end-grain of the base aluminum alloy material in the receiving opening surrounding and in contact with the rivet and smeared into position in the structural sheet member. However, this method of making the rivet is unsatisfactory in that such coating methods do not provide the proper anti-corrosion characteristics to the rivet itself. For this reason, a method of making the clad high-strength rivet and the resultant novel rivet is required.